The present invention relates to spread spectrum receivers and, more particularly, to a spread spectrum communication feed forward signal processor that uses a pilot carrier and a data carrier to reduce the effects of phase noise introduced by the transmission system.
Phase noise is a serious problem in commerical mobile satellite systems. Voice or data signals come from a telephone system into a fixed satellite terminal where they spread spectrum modulated onto a carrier and transmitted at Ku band to a satellite. The upconverter in this fixed station adds phase noise. The satellite receives the Ku band signal and translates it to L band. The translation is on the order of 10 GHz. In the vicinity of the mobile terminal the signal is scattered from trees and buildings, and the like, to add more phase noise due to mulitpath propagation. The mobile terminal receives the signal and translates it to baseband. All of these translations and the multipath propagation add phase noise. The total phase noise limits the system performance.
The phase noise problem is particularly severe where large frequency translations are used. In the above example, a 10 GHz carrier must be synthesized on the satellite to make the translation. This carrier is derived from a relatively low frequency crystal source (5 MHz). In the process of multiplying the frequency of the source, the phase noise of the source is also multiplied. The baseband to Ku band up-converter in the fixed uplink station also adds phase noise. However, higher purity sources are available for terrestrial application and the phase noise added to the link by the ground terminal is less than the phase noise added to the link by the satellite.
One problem with conventional spread spectrum communication receivers is that the phase noise of the pilot is added to the phase noise of the data signal, thereby degrading the performance.
In view of the disadvantages of conventional spread spectrum communication receivers, it was contemplated that a new and novel solution to the phase noise problem would process the signals in such a manner that the phase noise of the pilot would be substracted from the phase noise of the data signal, thus improving the performance. It was also contemplated that a novel solution to the phase noise problem would allow communication at lower signal-to-noise ratios on spread spectrum communication links corrupted by phase noise.